Logic Signal Standards Used to
Interconnect Instruments in SLRI
Logic Signal Standards in SLRI
• NIM (Nuclear Instrumentation Module)
Standard and Signals
• TTL (Transistor-Transistor Logic) Compatible
Signals
• Other signals: ECL (Emitter Coupled Logic), TTL
(real TTL ICs, not just compatible), CMOS etc.
Why we need this lecture
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TTL can mean two different things:
1. The term TTL is often used to describe a type of logic signal input or output
on stand alone instruments or NIM modules. These signals should properly be
called ‘TTL Compatible’ signals. The voltage logic levels of these signals are
compatible to those of TTL integrated circuits, i.e. they can be connected
together, input to output or output to input.
2. The term TTL is more properly used for Transistor-Transistor Logic integrated
circuits (ICs). It describes the logic voltage levels as well as the input and output
currents of TTL logic gates.
TTL compatible signal inputs and outputs can be connected to TTL integrated
circuit logic gate inputs and outputs – that’s why they are called ’compatible’.
But the input and output currents of TTL compatible connectors are not the
same as TTL integrated circuits as we shall see in this lecture. TTL compatible
outputs usually have much higher voltage (but max 5 V) and current capability
than TTL ICs and can often drive 50 ohm loads which TTL ICs cannot.
So whenever you see a connector on an instrument described as TTL you must
look in the manual to see if it is TTL Compatible or a real TTL IC input or output.
In particular see if an output can drive a 50 ohm load or not. Do NOT connect a
TTL IC output to 50 ohms, only to another TTL input.
What is NIM ?
The Nuclear Instrumentation Module (NIM) standard defines
mechanical and electrical specifications for electronics
modules used in experimental particle and nuclear physics. The
NIM standard was defined by the U.S. Atomic Energy
Commission in 1969 with the latest revision in 1990.
In SLRI, NIM is used by the Machine and Control groups (for
timing systems, etc), Safety Group (for radiation detection)
and most Beamlines (for radiation measurement and
spectroscopy).
Some Examples of SLRI Signal Types
Instrument
Digital Delay
Generators
Model
DG 535
Input/Output Signal Types
TTL, ECL and NIM compatible logic levels and variable logic levels from -3 to
+4 V. All can be terminated in either 50 Ohms or high impedance.
10MHz Ref in/out, 1Vpp square wave
NIM Modules
various
Linear analog pulse signals, of both + and - polarity, 0 - 10 V.
Positive Logic signals
Negative Logic signals
TTL compatible signal levels. Can be terminated in 50 ohm.
120 MHz Main
Oscillator
PTS 120
Analog RF, CW
TTL for control (HCT TTL), i.e. real TTL, not just compatible. Cannot be
terminated in 50 Ohms.
10MHz Ref out, RF sine wave, 0.4Vrms
The NIM Standard
The NIM (Nuclear Instrumentation Module) Standard defines mechanical and
electrical specifications for NIM electronic modules.
DC power needed
• The Bin must supply ±12 and ±24 volts DC power to the modules via a
backplane; the standard also specifies ±6 V DC and 220 V or 110 V AC pins,
but not all NIM bins provide them.
• http://en.wikipedia.org/wiki/Nuclear_Instrumentation_Module
Things You Should Know About NIM
• Only ±12 and ±24 volts DC power are needed in Bins for most NIM
module Spectroscopy applications, e.g. safety and beamline group
applications.
• ±6 V DC power is needed for most fast timing NIM modules.
• You can usually read the power needed on the front of the NIM
modules (see picture above).
• Measure the Bin on the front to see if it has ±6 V DC. Sometimes
there is a socket but no power.
• Check the voltage and ripple on the Bin front panel yearly with a
scope to make sure its good. Power supply capacitors might need
replacing.
• NIM modules should not be left for long periods (6 months or
more) without having the power turned on. Other wise the
Tantalum capacitors inside will loose their insulation properties and
short circuit when turned on. Power them up every 6 months.
NIM Standard Signals
• Linear Analog pulses, 0-10 V, proportional to the energy of a
detected radiation particle. µS timescale. Used by our Safety Group
and Beamlines.
• Slow positive logic pulses, µS timescale, 0-5V, similar to TTL. Not
used in our Control Room except for the radiation safety system.
• Fast negative logic pulses, nS timescale, now commonly called NIM
signals. These make up most of our Timing System signals.
NIM Positive Logic Signal
NIM positive logic is used for slow to medium speed logic signals, e.g. counters, with
repetition rates from dc to about 1MHz. The NIM standard defines positive logic signals
as follows
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Note that although this signal is normally about 5V it is NOT TTL compatible and
should not be connected to a TTL input because it can be more than 5V which
might damage a TTL input.
Before we discuss NIM Negative Logic Signals:
What is the difference between a Voltage and a Current
Signal ?
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Normally, in electronics, we deal with voltage signals, (measured with
scopes, DVMs, ADCs, etc). When anyone talks about a signal we always
assume it is a voltage signal unless they tell us it is not.
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This lecture, up till now, has dealt only with VOLTAGE signals not current
signals.
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But the NIM negative logic signal is a current signal.
The difference between voltage and current signals:
A large voltage in series with a large resistor can
make an approximate constant current source
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If R >> RL, then I = constant if RL changes
e.g. if RL = 0, and V = 10 volts and R = 10M ohms then I = 1uA
For RL= 10K ohm, I = V/(R + RL) = 10/(10.01) = 0.999 uA
• So a voltage source of high internal resistance can be
considered a constant current source of I = V/R
The NIM Negative Logic Current Signal
NIM Negative Logic Signals
NIM fast negative signals are used when the rise time or repetition rate
requirements exceed positive logic or TTL capability.
• Fast NIM rise times are typically about 2ns.
• The leading edge is used for triggering and timing purposes. The width is used for
gating and fast coincidence logic (AND/OR functions).
• All fast NIM signals MUST be terminated in just one 50 Ohm resistor, either inside
the receiving NIM module or with a 50 Ohm BNC terminator. The signal cables
cannot be split into two signals with BNC T’s.
• Only 50 Ohm cables must be used
The exact definition of the negative logic signal is a current signal as follows
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TTL Compatible Signals
• Most so called ‘TTL’ input or output signals in SLRI instruments are
actually just ‘TTL compatible’. That means they use the standard
TTL logic levels (voltages) but not the normal input or output
currents of TTL Integrated Circuit (IC) gates.
Standard TTL IC Logic Levels
• In addition ‘TTL compatible’ output signals can sometimes be
terminated in 50 Ohms unlike real TTL IC outputs. This is useful to
avoid reflections if the signal risetimes are fast. Check the manual
of the output instrument or NIM module first to avoid damage.
TTL (Transistor-Transistor Logic)
Integrated Circuits and Signal Levels
• Logic ‘0’ – must be less than 0.8 V
• Logic ‘1’ – must be more than 2 V
• Power supply, Vcc = +5 V (4.75 min to 5.25 max)
If you ever measure any TTL signals or power outside
these limits something is wrong and probably
broken.
Basic TTL NAND Gate
Real TTL IC circuits
NAND Gate 1
NAND Gate 2
Logic Symbols
Real TTL IC Output/Input currents for
Logic ‘0’
1.6 mA
OFF
ON
This transistor
must be able to
sink 1.6 mA. (or
16 mA for a
fanout of 10)
7400 NAND Gate 1
‘0’
7400 NAND Gate 2
1.6 mA
Logic Symbols
NAND Gate 1
NAND Gate 2
ECL (Emitter Coupled Logic)
(Real ECL, not just compatible logic levels)
Logic ‘1’
Logic ‘0’
ECL
• An ECL signal is like a fast NIM signal
offset by – 0.8 V
• But It is has two complementary
current outputs each giving a change
of about 8 mA into a 100 Ohm
terminating resisitor.
• 8mA x 100 ohms = 0.8V
• So ECL is a current based signal
One output
The other complimentary
output